Local area network (LAN) application focused on an information processing system has been dramatically developed. Among others, a network focused on the Ethernet (registered trademark) or the IEEE 802.3 Ethernet (hereinafter collectively referred to as the “Ethernet”) has been dramatically developed, and has spread not only to the information processing system but also to a monitoring control system. Further, due to improvement of performance such as a transmission speed and a communication processing function and price reduction in association with mass production of standard products, the network focused on the Ethernet has been surely developed.
The Ethernet having a 10 Mbps bus type transmission path and the Ethernet having a star type transmission path with a combination of a hub, a repeater, and a switching hub can be currently utilized by a combination of products with transmission speeds of 10 Mbps, 100 Mbps, and 1 Gbps.
Although future application of the Ethernet has been expected, a carrier sense multiple access/collision detection (CSMA/CD) method has been used as the method for controlling transmission of a communication frame among transmission stations under existing circumstances. Due to fundamental principles of such a method, a waiting time until the communication frame is reliably delivered onto the transmission path cannot be accurately predicted no matter how much the transmission speed increases.
Specifically, in the CSMA/CD method, each transmission station monitors a carrier on the transmission path, and starts delivering the communication frame when no communication frame is not delivered onto the transmission path for a certain period of time and there is a free space on the transmission path. On the other hand, in the CSMA/CD method, when there is no free space on the transmission path, delivery of the communication frame is delayed until the free space can be ensured on the transmission path. Further, in the CSMA/CD method, even when one transmission station starts delivering the communication frame, if occurrence of interference with another transmission station having simultaneously started delivering the communication frame is detected, delivery of the communication frame from the one transmission station is interrupted, and then, is resumed with a predetermined time delay.
Thus, in the CSMA/CD method, when the use rate of the transmission path by each transmission station increases, interference among the transmission stations is inevitably caused often. Further, the state in which the communication frame cannot be delivered lasts for all time. That is, in the CSMA/CD method, the waiting time until delivery of the communication frame is probabilistic. For this reason, employment of the CSMA/CD method is limited in the intended use for control requiring so-called real-time performance, such as the control of object movement or a process, by information exchange among transmission stations within a required time.
In the monitoring control system employing the CSMA/CD method, a countermeasure is also taken in such a manner that a system design is, regardless of high-speed transmission performance, made with such a sufficient allowance that occurrence of interference among the transmission stations can be ignored by reduction in a total traffic amount in information exchange among the transmission stations or the number of transmission stations in a transmission system, or made with further adjustment according to an operation status.
For realizing the real-time performance allowing reliable information exchange among the transmission stations within the certain period of time, a transmission system has been proposed, which is, by an implicit method not including explicit token exchange as in a token passing method represented by IEEE 802.4, configured as if the token passing method is realized.
In a transmission station (hereinafter referred to as a “data transmission station”) of this transmission system, an Ethernet transmission control Large Scale Integration (LSI) is utilized. A special signal pattern for the Ethernet transmission control is added to a frame format provided by the Ethernet. Using a synchronization frame delivered by a particular transmission station as the timing of starting token passing, each transmission station sequentially delivers a communication frame subsequently after the synchronization frame. This realizes a so-called implicit (implicit logic) token passing method in which occurrence of interference among the transmission stations as a drawback of the CSMA/CD method is reduced.
LSIs, connectors, cables, software resources, etc. forming the Ethernet can be utilized in this transmission system, but a signal sequence transmitted on a transmission path is different from the frame format provided by the Ethernet. For this reason, in a transmission system configured according to Ethernet standards, even if data transmission stations were mixed with Ethernet equipment such as a repeater, a hub, or a switching hub and transmission stations (hereinafter referred to as “Ethernet transmission stations”) according to the Ethernet standards, the implicit token passing method cannot be performed, and information exchange among the data transmission stations and the Ethernet transmission stations cannot be performed. That is, the transmission system including the data transmission stations leaves no other alternative but to serve as an independent transmission system for exclusive use.
For this reason, in a transmission system configured such that a plurality of transmission stations are connected together via a transmission path and each transmission station sends/receives a communication frame according to the Ethernet standards, a technique is employed, which can realize the implicit token passing method using some of the transmission stations as synchronization transmission stations configured to periodically output the timing of starting implicit token passing. According to such a technique, the data transmission stations can be connected to the transmission path including the Ethernet equipment. Thus, the transmission system can be provided with the real-time performance for realizing not only information exchange among the data transmission stations and the Ethernet transmission stations but also transmission of the communication frame within a preset time.
In the above-described technique, when an abnormality is caused in a repeater, a hub, a switching hub, a LAN controller, etc. on the transmission path, the abnormality in transmission of the communication frame might be detected in multiple transmission stations, and for this reason, it takes time to diagnose an actual abnormal portion. The method for solving such a problem may include a method in which an abnormality in various types of equipment of a transmission system is monitored by a simple network management protocol (SNMP).
However, in this method, a SNMP agent needs to be disposed at each transmission station in the transmission system. Moreover, transmission processing by the implicit token passing method is not taken into consideration, and therefore, this influences reliable transmission processing within a certain period of time. Further, the SNMP agent collects information in the transmission station at the timing different from the timing of transmitting the communication frame in the implicit token passing method. For this reason, when the abnormality is caused across multiple transmission stations, the actual abnormal portion cannot be diagnosed.